The
Control bar

Overview: The Control bar both changes and displays the
current axis, subsample rate and zoom factor.

Axis: The axis symbol shows the current
orientation of the axis. Clicking the axis symbol rotates the axis. Each
left click rotates the axis counter-clockwise 90 degrees. Each right click
rotates the axis clockwise 90 degrees.

Subsample: The subsample factor
determines the fraction of data points in the file that are displayed.
The subsample factor operates differently depending on whether the data
points are on a uniform grid or are on a non-uniform grid. For a uniform
grid every {subsample} data point is displayed. For example, if the
subsample factor is one, every data points is displayed; if it is
changed to ten,
every tenth data point is displayed. This results in a reduction, by a
factor of one hundred, in the total displayed data points.
For a non-uniform grid the program constructs a uniform grid which it uses as a mask on top of the
non-uniform grid. The details of how the program constructs this uniform
grid is given in the input file format section
of the docs. The program finds, for each square of the uniform grid, the
non-uniform grid data point that is closest to the center of each square
in the uniform grid. It then displays for each square of the uniform grid
that closest data point- but at the data point's location on the non-uniform
grid. What sample size affects in the case of a non-uniform grid, is the
relative coarseness of the uniform grid. A larger sample size will make
larger squares on the uniform grid that on average will contain more of
the non-uniform grid's data points and will thus result in less
vectors being displayed. A smaller sample size will make smaller squares
on the uniform grid that will on average contain fewer of the non-uniform
grid's data points and will thus result in more vectors being displayed.
The numeric value of sample size is the ratio of the size of a grid square
in the current uniform grid compared to the size of a grid square in the
original uniform grid i.e .5 means grid squares half the original size
resulting in a grid with four times as many points. Note also that the
arrow size is adjusted so that the arrow will fit inside the uniform grid
so that adjusting the subsample factor will also adjust the overall arrow
size. In addition note that a subsample factor of zero simply displays
every vector in the data.

Changing the Subsample factor: Two techniques using the control
bar can be used to change the subsample factor. The first technique
is to move the top scale bar until the subsample
factor is the desired value. The second technique is to simply left-click
on the number to the right of subsample:. The desired subsample
factor can then be typed in. The subsample factor can also be changed using
the keyboard.

Arrow mag: Arrow mag allows the
user to change the arrow display size in mmDisp. Increasing the arrow
magnitude value will draw longer arrows on the display. Decreasing
the arrow magnitude value will draw shorter arrows on the display.

ZOOM: The zoom controls the magnification
of the vector display. Assuming no other changes, increasing the zoom factor
results in a smaller region of data displayed in the same size window with
a corresponding increase in arrow size. However the size of the arrow,
as discussed below, depends also on the subsample rate and the size of
the frame. The zoom factor is the number of pixels in the frame
divided by the number of arrows in the frame assuming the subsample
rate is one. When the subsample rate is increased from one, the number
of arrows is decreased and the number of pixels available to each arrow
is increased causing an increase in the arrow's size. Thus the subsample
rate also effects the size of the arrows but in a way independent of the
zoom factor. The subsample rate corresponds to the resolution of the display
while the zoom factor corresponds to the closeness to the display.

Changing the Zoom level: Two techniques using the control bar,
one technique using the keyboard, and
one technique that works by outlining an area
in the display window, can be used to change the zoom level. The first
technique is to move the bottom scale bar
until the zoom factor is the desired value. The second technique
is to simply left-click on the number to the right of Zoom:. The
desired zoom value can then be typed in. A menu command, Fill
Display , can also change the zoom level.

Turning Off: The control bar may be turned off by toggling
the control bar option off under the option menu.